321 lines
12 KiB
HTML
321 lines
12 KiB
HTML
<html>
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<title>Mesa EGL</title>
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<head><link rel="stylesheet" type="text/css" href="mesa.css"></head>
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<body>
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<h1>Mesa EGL</h1>
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<p>The current version of EGL in Mesa implements EGL 1.4. More information
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about EGL can be found at
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<a href="http://www.khronos.org/egl/" target="_parent">
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http://www.khronos.org/egl/</a>.</p>
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<p>The Mesa's implementation of EGL uses a driver architecture. The main
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library (<code>libEGL</code>) is window system neutral. It provides the EGL
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API entry points and helper functions for use by the drivers. Drivers are
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dynamically loaded by the main library and most of the EGL API calls are
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directly dispatched to the drivers.</p>
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<p>The driver in use decides the window system to support.</p>
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<h2>Build EGL</h2>
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<ol>
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<li>
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<p>Run <code>configure</code> with the desired client APIs and enable
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the driver for your hardware. For example</p>
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<pre>
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$ ./configure --enable-gles1 --enable-gles2 \
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--with-dri-drivers=... \
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--with-gallium-drivers=...
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</pre>
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<p>The main library and OpenGL is enabled by default. The first two options
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above enables <a href="opengles.html">OpenGL ES 1.x and 2.x</a>. The last two
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options enables the listed classic and and Gallium drivers respectively.</p>
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</li>
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<li>Build and install Mesa as usual.</li>
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</ol>
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<p>In the given example, it will build and install <code>libEGL</code>,
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<code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>, and one
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or more EGL drivers.</p>
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<h3>Configure Options</h3>
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<p>There are several options that control the build of EGL at configuration
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time</p>
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<ul>
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<li><code>--enable-egl</code>
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<p>By default, EGL is enabled. When disabled, the main library and the drivers
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will not be built.</p>
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</li>
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<li><code>--with-egl-driver-dir</code>
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<p>The directory EGL drivers should be installed to. If not specified, EGL
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drivers will be installed to <code>${libdir}/egl</code>.</p>
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</li>
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<li><code>--enable-gallium-egl</code>
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<p>Enable the optional <code>egl_gallium</code> driver.</p>
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</li>
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<li><code>--with-egl-platforms</code>
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<p>List the platforms (window systems) to support. Its argument is a comma
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seprated string such as <code>--with-egl-platforms=x11,drm</code>. It decides
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the platforms a driver may support. The first listed platform is also used by
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the main library to decide the native platform: the platform the EGL native
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types such as <code>EGLNativeDisplayType</code> or
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<code>EGLNativeWindowType</code> defined for.</p>
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<p>The available platforms are <code>x11</code>, <code>drm</code>,
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<code>fbdev</code>, and <code>gdi</code>. The <code>gdi</code> platform can
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only be built with SCons. Unless for special needs, the build system should
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select the right platforms automatically.</p>
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</li>
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<li><code>--enable-gles1</code> and <code>--enable-gles2</code>
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<p>These options enable OpenGL ES support in OpenGL. The result is one big
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internal library that supports multiple APIs.</p>
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</li>
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<li><code>--enable-shared-glapi</code>
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<p>By default, <code>libGL</code> has its own copy of <code>libglapi</code>.
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This options makes <code>libGL</code> use the shared <code>libglapi</code>. This
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is required if applications mix OpenGL and OpenGL ES.</p>
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</li>
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<li><code>--enable-openvg</code>
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<p>OpenVG must be explicitly enabled by this option.</p>
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</li>
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</ul>
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<h2>Use EGL</h2>
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<h3>Demos</h3>
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<p>There are demos for the client APIs supported by EGL. They can be found in
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mesa/demos repository.</p>
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<h3>Environment Variables</h3>
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<p>There are several environment variables that control the behavior of EGL at
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runtime</p>
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<ul>
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<li><code>EGL_DRIVERS_PATH</code>
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<p>By default, the main library will look for drivers in the directory where
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the drivers are installed to. This variable specifies a list of
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colon-separated directories where the main library will look for drivers, in
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addition to the default directory. This variable is ignored for setuid/setgid
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binaries.</p>
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<p>This variable is usually set to test an uninstalled build. For example, one
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may set</p>
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<pre>
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$ export LD_LIBRARY_PATH=$mesa/lib
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$ export EGL_DRIVERS_PATH=$mesa/lib/egl
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</pre>
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<p>to test a build without installation</p>
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</li>
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<li><code>EGL_DRIVER</code>
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<p>This variable specifies a full path to or the name of an EGL driver. It
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forces the specified EGL driver to be loaded. It comes in handy when one wants
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to test a specific driver. This variable is ignored for setuid/setgid
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binaries.</p>
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</li>
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<li><code>EGL_PLATFORM</code>
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<p>This variable specifies the native platform. The valid values are the same
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as those for <code>--with-egl-platforms</code>. When the variable is not set,
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the main library uses the first platform listed in
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<code>--with-egl-platforms</code> as the native platform.</p>
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<p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
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create displays for non-native platforms. These extensions are usually used by
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applications that support non-native platforms. Setting this variable is
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probably required only for some of the demos found in mesa/demo repository.</p>
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</li>
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<li><code>EGL_LOG_LEVEL</code>
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<p>This changes the log level of the main library and the drivers. The valid
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values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
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<code>fatal</code>.</p>
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</li>
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<li><code>EGL_SOFTWARE</code>
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<p>For drivers that support both hardware and software rendering, setting this
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variable to true forces the use of software rendering.</p>
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</li>
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</ul>
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<h2>EGL Drivers</h2>
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<ul>
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<li><code>egl_dri2</code>
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<p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
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It functions as a DRI driver loader. For <code>x11</code> support, it talks to
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the X server directly using (XCB-)DRI2 protocol.</p>
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<p>This driver can share DRI drivers with <code>libGL</code>.</p>
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</li>
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<li><code>egl_gallium</code>
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<p>This driver is based on Gallium3D. It supports all rendering APIs and
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hardwares supported by Gallium3D. It is the only driver that supports OpenVG.
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The supported platforms are X11, DRM, FBDEV, and GDI.</p>
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<p>This driver comes with its own hardware drivers
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(<code>pipe_<hw></code>) and client API modules
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(<code>st_<api></code>).</p>
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</li>
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<li><code>egl_glx</code>
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<p>This driver provides a wrapper to GLX. It uses exclusively GLX to implement
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the EGL API. It supports both direct and indirect rendering when the GLX does.
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It is accelerated when the GLX is. As such, it cannot provide functions that
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is not available in GLX or GLX extensions.</p>
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</li>
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</ul>
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<h2>Packaging</h2>
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<p>The ABI between the main library and its drivers are not stable. Nor is
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there a plan to stabilize it at the moment. Of the EGL drivers,
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<code>egl_gallium</code> has its own hardware drivers and client API modules.
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They are considered internal to <code>egl_gallium</code> and there is also no
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stable ABI between them. These should be kept in mind when packaging for
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distribution.</p>
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<p>Generally, <code>egl_dri2</code> is preferred over <code>egl_gallium</code>
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when the system already has DRI drivers. As <code>egl_gallium</code> is loaded
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before <code>egl_dri2</code> when both are available, <code>egl_gallium</code>
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is disabled by default.</p>
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<h2>Developers</h2>
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<p>The sources of the main library and the classic drivers can be found at
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<code>src/egl/</code>. The sources of the <code>egl</code> state tracker can
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be found at <code>src/gallium/state_trackers/egl/</code>.</p>
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<p>The suggested way to learn to write a EGL driver is to see how other drivers
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are written. <code>egl_glx</code> should be a good reference. It works in any
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environment that has GLX support, and it is simpler than most drivers.</p>
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<h3>Lifetime of Display Resources</h3>
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<p>Contexts and surfaces are examples of display resources. They might live
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longer than the display that creates them.</p>
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<p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
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display resources should be destroyed. Similarly, when a thread is released
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throught <code>eglReleaseThread</code>, all current display resources should be
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released. Another way to destory or release resources is through functions
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such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
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<p>When a resource that is current to some thread is destroyed, the resource
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should not be destroyed immediately. EGL requires the resource to live until
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it is no longer current. A driver usually calls
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<code>eglIs<Resource>Bound</code> to check if a resource is bound
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(current) to any thread in the destroy callbacks. If it is still bound, the
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resource is not destroyed.</p>
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<p>The main library will mark destroyed current resources as unlinked. In a
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driver's <code>MakeCurrent</code> callback,
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<code>eglIs<Resource>Linked</code> can then be called to check if a newly
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released resource is linked to a display. If it is not, the last reference to
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the resource is removed and the driver should destroy the resource. But it
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should be careful here because <code>MakeCurrent</code> might be called with an
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uninitialized display.</p>
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<p>This is the only mechanism provided by the main library to help manage the
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resources. The drivers are responsible to the correct behavior as defined by
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EGL.</p>
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<h3><code>EGL_RENDER_BUFFER</code></h3>
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<p>In EGL, the color buffer a context should try to render to is decided by the
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binding surface. It should try to render to the front buffer if the binding
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surface has <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_SINGLE_BUFFER</code>; If the same context is later bound to a
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surface with <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_BACK_BUFFER</code>, the context should try to render to the back
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buffer. However, the context is allowed to make the final decision as to which
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color buffer it wants to or is able to render to.</p>
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<p>For pbuffer surfaces, the render buffer is always
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<code>EGL_BACK_BUFFER</code>. And for pixmap surfaces, the render buffer is
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always <code>EGL_SINGLE_BUFFER</code>. Unlike window surfaces, EGL spec
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requires their <code>EGL_RENDER_BUFFER</code> values to be honored. As a
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result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
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<code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
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config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
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pbuffer surfaces.</p>
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<p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
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single-buffered, in that <code>eglSwapBuffers</code> has no effect on them. It
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is desirable that a driver allocates a private color buffer for each pbuffer
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surface created. If the window system the driver supports has native pbuffers,
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or if the native pixmaps have more than one color buffers, the driver should
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carefully attach the native color buffers to the EGL surfaces, re-route them if
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required.</p>
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<p>There is no defined behavior as to, for example, how
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<code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>. Right
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now, it is desired that the draw buffer in a client API be fixed for pixmap and
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pbuffer surfaces. Therefore, the driver is responsible to guarantee that the
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client API renders to the specified render buffer for pixmap and pbuffer
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surfaces.</p>
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<h3><code>EGLDisplay</code> Mutex</h3>
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The <code>EGLDisplay</code> will be locked before calling any of the dispatch
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functions (well, except for GetProcAddress which does not take an
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<code>EGLDisplay</code>). This guarantees that the same dispatch function will
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not be called with the sample display at the same time. If a driver has access
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to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
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should as well lock the display before using it.
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</body>
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</html>
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